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Version: 2.3.2

Subscriptions

tip

Only Enterprise Edition supports

The data can be replicated to another CnosDB cluster by subscription, and the data replication will help to improve the fault tolerance and reliability of the whole system. CnosDB supports managing subscriptions through SQL, and CnosDB supports subscribing through Telegraf or another CnosDB cluster.

Create Subscription​

We can use CREATE SUBSCRIPTION to create a subscription.

Syntax​

CREATE SUBSCRIPTION <subscription_name> ON <database_name> DESTINATIONS ALL "<host_nmae>" ["<host_name>"]

Note:

  1. host_name indicates the host_name of the grpc service of the CnosDB node that subscribing to this node. All the data written to the specified database, CnosDB, will be copied and distributed to the host node.
  2. ALL Indicates the data replication mode. Currently, only ALL is supported.

Example​

For example, the partial configuration of the CnosDB node that accepts the distributed data is as follows:

[cluster]
meta_service_addr = ["127.0.0.1:8901"]

grpc_listen_port = 8903

The SQL for creating the subscription in the current CnosDB is as follows:

CREATE SUBSCRIPTION test ON public DESTINATIONS ALL "127.0.0.1:8903"

At this time, if any data is written to the current CnosDB node, the data will be synchronized copied and forwarded to 127.0.0.1:8903.

Alter Subscription​

We can use ALTER SUBSCRIPTION to alter the subscription.

Syntax​

ALTER SUBSCRIPTION <subscription_name> ON <database_name> DESTINATIONS ALL "<host_name>" ["<host_name>"]

Example​

ALTER SUBSCRIPTION test ON public DESTINATIONS ALL "127.0.0.1:8903" "127.0.0.1:8913"

You can modify host_name in this way, and note that modifying by ALTER SUBSCRIPTION overwrites it directly. If you do not want to delete host_name, ALL previous host_name needs to be added after DESTINATIONS ALL.

Show Subscription​

We can use SHOW SUBSCRIPTION to show the subscription information.

Syntax​

SHOW SUBSCRIPTION ON <database_name>

Example​

SHOW SUBSCRIPTION ON public
SUBSCRIPTION,DESTINATIONS,MODE
test,"127.0.0.1:8902,127.0.0.1:8903",ALL

Drop Subscription​

We can use DROP SUBSCRIPTION to drop the subscription.

Syntax​

DROP SUBSCRIPTION <subscription_name> ON <database_name>

Example​

DROP SUBSCRIPTION test ON public

Subscription Distribution Through Telegraf​

Install Telegraf​

You can refer to Telegraf to know how to use Telegraf and how to install Telegraf.

Telegraf Configuration​

Supposing that we've started CnosDB and created a subscription, set DESTINATIONS to 127.0.0.1:8803 :

> SHOW SUBSCRIPTION ON public;
+--------------+----------------+-------------+
| Subscription | DESTINATIONS   | Concurrency |
+--------------+----------------+-------------+
| sub_tr_1003  | 127.0.0.1:8803 | ALL         |
+--------------+----------------+-------------+

Add the input plug-in cnosdb in the configuration file of Telegraf, and configure the listening address and port number as follows:

[[inputs.cnosdb]]
service_address = ":8803"

Configure the output plug-in http to distribute messages. If there is another CnosDB instance with an HTTP listening port number of 127.0.0.1:8912, we can forward the subscription message to that instance with the following configuration.

[[outputs.http]]
url = "http://127.0.0.1:8912/api/v1/write?db=destination"
timeout = "5s"
method = "POST"
username = "admin"
password = "admin"
data_format = "influx"
use_batch_format = true
content_encoding = "identity"
idle_conn_timeout = 10